New technologies such as coordinated multi-point (CoMP), interference alignment (IA), dirty paper coding (DPC), massive multiple-input multiple-output (MIMO), etc. may be some of the keys to capacity enhancement for wireless systems. However, all of the benefits provided by these technologies may not be realized due to the requirements for precise channel knowledge. For a frequency division duplex (FDD) system, various channel feedback schemes have been proposed. However, the overhead, accuracy, and feedback delay are still major roadblocks.